Wideband multifunction radars are capable of concurrently performing hemispheric surveillance, tracking and simultaneously illuminating multiple targets in diverse environments. It is widely recognized that only active phased array antenna and radar systems with their inherent waveform flexibility, high stability and beam switching speed can successfully cope with this broad mission.
For the control of phased array radars, photonic architectures can be broadly characterized as either optically coherent or non-coherent. Although optically coherent architectures have been laboratory demonstrated on a limited scale, their application to a tactical system, where thousands of optical signals must be phase locked is not practical.
The performance issues facing active phased array radars are radio frequency (RF) bandwidth (shared multifunction apertures, imaging, adaptive nulling), true time delay steering (wide instantaneous bandwidth), electromagnetic interference (EMI) and beam steering control. Realizable active arrays providing this performance are limited in weight and size and are generally costly. In particular, transmit/receive T/R modules and array substructures are key cost drivers.
In accordance with the invention, photonic technology is applied to phased array radar systems. Preferably, the invention reduces cost, weight and size, while mitigating EMI, accommodating wider signal bandwidths and providing frequency independent beam steering of simultaneous multiple beams spanning multiple radar bands via the generation of true time delays. Solid state radar systems, airborne systems and shipboard systems can benefit from the invention.
The radar system comprises a plurality of subarrays of antenna elements and a plurality of optical carrier signals. Each antenna element belongs to a selected subarray and each optical carrier signal is within a unique, non-overlapping frequency band. A modulator modulates each optical carrier signal by a transmit radar signal. A time delay system employs wavelength division multiplexing of the modulated optical signals for each antenna element so as to direct a radar beam pattern from the array of antenna elements. A preferred embodiment of the invention is a planar array radar system having a true time delay wavelength division multiplexing architecture.
The radar array in accordance with the invention preferably includes N elements divided into M subarrays with n elements per subarray. A plurality of M tunable, single wavelength optical sources, with wavelengths xcex1 through xcexM, correspond to an element in each of the M subarrays. Other elements of the radar system include bi-directional photonic links, multiplexing to reduce parts count, and true time delay for all elements.
Beginning with the transmit function of the array, a transmission signal is amplitude modulated onto the carrier optical signals. After modulation, a star coupler multiplexes the M modulated optical signals onto M fibers, where they are time delayed, t1 through tM via a dispersive optical delay line. These M time delays represent the relative delays between the elements of each of the M subarrays. Each of the optical signals then require an additional time delay of T1 through TM from binary non-dispersive Time Delay Units (TDU) to create a linear phase front. These M time delays adjust for the relative offsets between subarrays.
The optical output signals are then split n times, filtered and distributed to the n elements in the corresponding subarray. The optical filters are tuned to select the time delay corresponding to the element location within a subarray. That is, the optical filter for the mth element of each subarray is tuned to pass the optical signal xcexm and reject the others. At the array, a photodiode removes the time delayed microwave signal from the optical carrier, and upon amplification, the microwave signal is transmitted.
For the receive function of the architecture, the microwave signal is routed, in reverse, through the signal chain. The modulated optical signals from a subarray are combined on a single fiber, and acquire the corresponding subarray time delays T1 through tM. The signals from a subarray are then divided and filtered in the same manner as for transmit. After filtering, only M2 modulated optical signals with the proper time delays remain. Prior to combining, these signals are attenuated to realize the desired array amplitude taper on receive.
To avoid the problems associated with coherent combining, a specific non-coherent optical combiner is utilized. This device, through a photodetector array, demodulates the links and recovers the coherent sum of the RF signals. Preferably, there is one photodetector for each antenna element in the radar array. A phase shifter can also be used to introduce a phase shift into selected photodetector outputs. In a particular preferred embodiment, the photodetectors are fabricated as Metal-Semiconductor-Metal devices on a common substrate.